In this application, the terms gypsum, FGD, calcium sulfate, calcium sulfite, calcium sulfate dihydrate, calcium sulfate hemihydrate, alpha gypsum plaster, alpha calcium sulfate hemihydrate, beta calcium sulfate hemihydrate and calcium sulfate anhydride are used. These various forms of calcium containing compounds require definition.
Gypsum is a mineral having the formula CaSO.sub.4.2H.sub.2 O, which in pure form or when the number of water molecules in the crystal is to be emphasized, is referred to as calcium sulfate dihydrate (CAS Registry number 10101-41-4). About 55 million tonnes of gypsum are used in industry each year. About one-half is processed by partial dehydration to beta calcium sulfate hemihydrate, .beta.CaSO.sub.4.0.5H.sub.2 O (CAS registry No 10034-76-1) commonly called "plaster of Paris". The beta calcium sulfate hemihydrate is used in the wallboard industry. At present, almost all of the commercial material is derived from mineral sources. This commercial product is never pure but has a minimum of 80% .beta.CaSO.sub.4.0.5H.sub.2 O. Complete dehydration produces calcium sulfate anhydrite (or simply "anhydrite") CaSO.sub.4 (CAS registry No. 7778-18-9).
Synthetic gypsum, CaSO.sub.4.2H.sub.2 O, (as opposed to the "natural" mineral gypsum), is available in North America in very large quantities, although until recently it was not used by the Gypsum Industry. Objections to its use arose because of the presence of impurities, its undesirable very fine physical state, its production in a wet state and its different handling characteristics. Synthetic gypsum is produced as a by-product from chemical processes, such as stack gas scrubbing. The waste from stack gas scrubbing contains calcium sulfite (CaSO.sub.3), calcium sulfate (CaSO.sub.4.2H.sub.2 O), and residual limestone (CaCO.sub.3). It is produced as a slurry commonly referred to as FGD (flue gas desulfurization) wastes.
The thermal dehydration of "natural" gypsum to produce calcium sulfate hemihydrate has been the subject of much theoretical and practical study. Two forms of the hemihydrate have been identified. The most common is aforesaid .beta.CaSO.sub.4.0.5H.sub.2 O which is produced industrially by dehydration at about 100.degree. C. The other form is aforesaid alpha, which is produced under conditions above 97.degree. C. in saturated steam. Although chemically identical to the .beta. form, this .alpha.CaSO.sub.4.0.5H.sub.2 O is considered a specialty product and is referred to as "alpha plaster". The importance of the difference in form and behaviour of these two hemihydrates may be appreciated by the 1992 market values. .beta.CaSO.sub.4.0.5H.sub.2 O commanded a price of about US$16 per short ton while CaSO.sub.4.5H.sub.2 O reached US$300 per short ton. Despite the difference in price, there is still some controversy in the literature in defining the form.
A practical method of distinguishing the two forms has been to use the amount of water required to make a flowable paste from 100g of a hemihydrate. The alpha form has a "low" water demand of 30g-40g, while the beta form has a relatively higher water demand of 75g-90g. In addition to this practical water demand test, Differential Scanning Calorimetry (DSC) may be used as an analytical technique to distinguish the two forms. In the test, a sample is heated and the amount of heat absorbed and released by the sample is measured. The alpha form displays a characteristic exothermic peak on the chromatogram suitable for identifying and roughly quantifying the amount of alpha in the sample. One characteristic of the alpha form is that the form has larger, squat or columnar crystals as compared to the tiny, irregular crystals of the beta form. There is also a needle-like crystal habit of the alpha form that does not have the desired low water demand but which is characterized as alpha in the DSC tests.
Thus, in this specification and claims the term alpha calcium sulfate hemihydrate means the squat, columnar crystals of calcium sulfate hemihydrate that have the low water demand of &lt;40g water per 100g solids and have the characteristic isotherm trace on a differential scanning calorimetry (DSC) thermogram.
Markets for alpha plaster make use of the low water demand characteristic to obtain the dense, high strength, hydrated gypsum product produced therefrom. In North America, between 150,000 and 200,000 short tons are produced which is mainly used in moulding and casting plasters, dental plasters, art and statuary plasters, pattern and model making and in self-levelling floor mortars and well-castings. We believe that the growth of the alpha plaster market has been limited by its cost and not its desirable properties.
The commercial processes for producing alpha plaster use autoclave technology. One process consists of applying steam at a pressure of 117 kPa at 123.degree. C. for 5-7 hours on lumps of natural gypsum. A concentrated magnesium-containing solution or slurry autoclave process was described by Schoch and Cunningham in 1940 ("Production of Gypsum plaster by Wet Methods" Meeting of the American Society of Chemical engineers New Orleans La. (1940). U.K. Patent No 1,079,502 published Aug. 16, 1967 describes use of a crystal-modifying succinic or malic acid to produce the alpha form from a slurry autoclave system. Hoggatt, U.S. Pat. No. 2,616,789, published Nov. 4, 1952 describes a solution process which does not use pressure but instead uses a concentrated calcium chloride (30 wt %) to allow temperatures above 120.degree. C. to be obtained without an autoclave.
Production of alpha plaster using acids has been known for some time. Schoch and Cunningham (ibid) reported dehydration by nitric acid. Research and pilot plant work using sulfuric acid is described in "Construction and Operation of a Pilot Plant for the production of High Quality Stucco for Mine Mortars", Research Report, BMFT-FB-86-088, Deuster D. German Ministry for Research and Technology, Dec. 1986). This process was not successful because it is extremely difficult to produce alpha plaster without considerable amounts of anhydride. Hemihydrate can only be formed in a narrow range of temperatures and acid concentrations and outside of this range, formation of anhydride can be practically instantaneous (Kuntze R. A. et al "Utilization of Waste derived Gypsum for Mine Backfill" International Symposium on Tailings and Effluent Management, Halifax NS Aug.1989).
Conditions for production of alpha plaster from natural gypsum do not work for FGD wastes. Commercial production of alpha plaster from FGD wastes has, thus, as yet not been successful.
FGD sludge waste containing calcium sulfite has been disposed of in several ways. It can be used to substitute for natural gypsum after being converted into calcium sulfate dihydrate by oxidization with air. This application is limited by a low market demand. It is also common to dispose of FGD waste by sending it to landfill. However, sulfite in the FGD waste can be oxidized and thus, consume oxygen to pose a toxicity threat. Accordingly, if sulfite leaches into groundwater, oxygen is consumed and aquatic species, such as fish may die. Further, disposal by landfill increases the overall cost of flue gas desulfurization because of tipping fees.
Basically, processes for producing .alpha.-HH are roughly classified into three types. They are steam autoclaving, slurry autoclaving and solution processes at atmospheric pressures. The so-called solution process has produced .alpha.-HH by suspending gypsum powder in an aqueous solution containing relatively concentrated inorganic/organic salts and acids and heating the resulting suspension to the boiling point of the aqueous solution at atmospheric pressure for a time sufficient to complete conversion of DH to .alpha.-HH. The limitations of the reported solution processes are that very concentrated salt solutions are required to raise the boiling point; that the concentrated solutions are difficult to removal from the product; that the concentrated solutions require very careful process control since the product can quickly dehydrate to the undesirable anhydride form; that the concentrated solutions are corrosive; that the concentrated solutions add expense to the process; and further that the concentrated solutions create a waste water disposal problem.
In general, the salts and acids added have physicochemical properties such as lowering the water partial pressure or equivalently increasing the boiling point of the solution, catalyzing the dehydration of gypsum, modifying crystal habit and promoting the rate of growth of the crystals. As is well known in the literature, a number of substances have been proposed to be used in the solution for transforming gypsum and also to a more limited extent, synthetic gypsum to .alpha.-HH. These additives include magnesium sulfate, magnesium chloride, calcium chloride, sodium chloride, sulfuric acid, nitric acid, phosphoric acid and the like as well as some alkali and alkaline earth metal salts of organic acids, such as calcium alkylaryl sulphonate, and magnesium sulfosuccinate salts. Besides the above mentioned substances, some organic acids and salts thereof have been suggested as crystal habit modifiers in low concentrations (0.1%-1%) in order to obtain the desired crystal habit. Succinic, citric and sulfosuccinic acid and salts thereof have been suggested for this effect. However, the limitations of the concentrated solutions remain and solutions suitable for use by industry remain an elusive goal.
To obtain good crystals of .alpha.-HH, generally the residence time of solids in the solution has to be long enough to allow crystal growth. Unfortunately, .alpha.-HH is not stable in aqueous solution and the time required to grow large crystals in those solutions reported in the prior art, particularly in sulfuric acid solutions is too short to be of commercial interest. Further the stability of .alpha.-HH is extremely sensitive to the changes in both composition and temperature of the solution. It has been reported that .alpha.-HH is only stable for a short period in a sharply narrow range of either the temperature or the composition of the heating media.
There, thus, remains a significant demand for a process that provides for the economic manufacture of .alpha.-HH.